1. Field of the Invention
The present invention relates to drive circuits for liquid crystal displays, and more particularly, to drive circuits that reduce kickback voltage and power consumption.
2. Description of the Related Art
Liquid crystal displays (LCD's) have grown increasingly popular as substitutes for cathode ray tubes in electronic appliances. LCD's can be driven by large scale integrated circuits because of their low-voltage and low-power consumption characteristics. Accordingly, LCDs have been widely produced on a commercial scale for use in laptop computers, pocket computers, automobiles, color televisions, etc.
Thin film transistor liquid crystal displays (TFT-LCDs) typically use twisted nematic (TN) crystals and have a transistor and a storage capacitor associated with each pixel. The transistor and capacitor are made of a thin film, such as amorphous silicon on a glass substrate. A pixel in a TFT-LCD can only be turned on by applying a gate signal to the transistor associated with the pixel. A display that uses transistors to turn pixels on and off is referred to as an active display.
FIG. 1 shows a schematic diagram of a typical TFT-LCD array. Each pixel circuit, or cell, includes a switching transistor, a liquid crystal, and a storage capacitor. Matrix addressing is provided by data lines which connect the source terminals of transistors in each column of cells, and gate lines which connect the gates of transistors in each row of cells. The liquid crystal in each cell is connected between the drain terminal of the transistor and a common electrode, and the storage capacitor is connected between the drain terminal of the transistor and the gate line of the previous row. A pixel is selected by activating a data line and applying a gate signal to a gate line. Since transistors can be selected individually in a TFT-LCD, there is no cross talk between pixels. The storage capacitor stores an electric charge so that the state of the pixel is maintained during a non-selected period.
Referring to FIG. 1, when switching transistors TFT1 and TFT2 are turned on, capacitors Cst1 and Cst2 receive a charge and liquid crystals Clc1 and Clc2 display a grey level based on the voltage level applied to the source terminals of transistors TFT1 and TFT2 through the data line. When the transistors are turned off, the capacitors maintain the voltage level on crystals Clc1 and Clc2, which also have some parasitic capacitance. As long as the leakage current through the crystals is not excessive, the grey level of the crystals is maintained until the pixel is refreshed during the next frame update.
The voltage versus current characteristic of switching transistors TFT1 and TFT2 is shown in FIG. 2.
LCD cells typically require a net DC bias of zero volts to avoid electrochemical degradation. A common method for minimizing the net DC voltage is to apply an AC square wave, typically having a magnitude of 5 volts, to the common electrode terminal, or backplane. The gates of the transistors are then driven with a gate signal having a gate drive signal superimposed on a square wave as is shown in FIGS. 3A and 3B. The square wave portion of the gate signal indicated by Voff1 and Voff2 is the off time. Because the gate signal is in phase with the square wave signal on the common electrode terminal, no voltage is applied to the gate of the switching transistors during the off time. The portion of the gate signal indicated by Von1 and Von2 are applied to the gates lines at intervals of one frame and drive the switching transistors with grey level voltages.
A major problem with this technique is that a high kickback voltage is generated due to the parasitic capacitance Cgs in the switching transistors. When the gate signal changes from Von to Voff, the electric charge in the liquid crystals Clc1 and Clc2 or the capacitors Cst1 and Cst2 is partially transferred to the parasitic capacitance Cgs. As a result, a drop in the grey level voltage is produced. This drop in the grey level voltage is called as the kickback voltage Vk and is given by: EQU Vk=Cgs(Von-Voff)/(Cgs+Clc+Cst) Eq(1)
If the kickback voltage Vk has a high value, the kickback voltage is applied to the TFT-LCD, thereby increasing power consumption and causing poor images due to flickering, stitching, etc.
Accordingly, a need remains for a drive circuit for a liquid crystal display that overcomes the problems described above.